An electrophoretic display generally comprises a suspension of charged pigment particles colloidally dispersed in a liquid of matching specific gravity contained in a cell comprising two parallel and transparent conducting electrode panels. The charged particles are transported between the electrode panels under the influence of an electric field. In one embodiment of an electrophoretic display disclosed in Hou, U.S. Pat. No. 5,298,833, black particles are suspended in a back-lighted clear medium. When the black particles are moved against the front transparent electrode, there is displayed a black image contrasted against a light background. Unfortunately, such a display requires a back light, and therefore it is not desirable for notebook computers due to power and weight requirements of the backlight.
Dalisa, "Electrophoretic Display Technology", Proceedings of S.I.D., Vol. 18, No. 1, 1977, pp. 43-50; and "Electrophoretic Displays", Topics of Applied Physics, Vol. 40, 1980, pp. 213-232, discloses a reflective electrophoretic display (e.g., no back light) comprising charged pigment dispersed in a dyed liquid of contrasting color enclosed in a cell comprising parallel and transparent conducting front and rear electrode panels. When the pigment is packed on the front electrode panel, the color of the pigment will be seen by the observer with ambient light. When the pigment is on the rear electrode panel, the ambient room light is absorbed and scattered by the dyed liquid and the color of the dye is observed. Because the display is passive and requires no back light, the display consumes very little power. To achieve the greatest contrast, light particles with large scattering coefficients such as titanium dioxide are suspended in a dark dyed medium. Displays utilizing this approach, however, do not produce highly contrasted images and generally have low reflectivity. When their dye concentration is low, the suspension, and hence the background, appears grayish. When their dye concentration is high, the light state is not bright because the dark dye is easily noticed in the spaces between the particles and in the spaces between the particles and the viewing plate.
Therefore, there is a continuing need in the art for an electrophoretic display having low power consumption and good contrast and brightness.
It is therefore an object of the present invention to provide an improved electrophoretic display having low power consumption and good image contrast. Other objects and advantages will become apparent from the following disclosure.